Atlas of plant and animal histology

Pacini corpuscle: it is a type of encapsulated receptor found in the deep dermis, pleura, nipples, pancreas, tendons, penis and clitoris, and in deeper parts of urinary bladder and joints. Pacini corpuscles are activated with fast movements such as vibrations and mechanical strong pressures. Their size and morphology is variable. For instance, it looks like the deeper they are located the larger they are. Similar crpuscles, but more simple than Pacini corpuscles, are found in the fingertips, which are referred as Golgi-Manzone corpuscles.

Palate: it is the roof of the oral cavity.

Palisade parenchyma: (in plants) it is the photosynthetic parenchyma found below the epidermis of the upper part of leaves. Palisade parenchyma is made up of parenchymatic cells arranged tightly packaged with their major axis oriented perpendicularly to the leaf surface.

♣ Plant tissues: Parenchyma, Photosynthetic parenchyma .

♣ Plant organs: Leaf, Pine leaf , Dicot leaf .

Palmate leaf: (in plants) it is a type of compound leaf with more than one main vein. These veins are organized as spokes radiating from a common point.

Pancreas: it is an internal organ of animals, found near the stomach. The pancreas is composed of two domains: an exocrine domain consisting in the pancreatic acini that release their content (digestive enzymes) into the digestive duct, and an endocrine domain made up of the Langerhans islets that release hormones (insulin and glucagon) and end up in the circulatory system.

♣ Animal tissues: Glandular epithelium, Pancreas .

♣ Animal organs: Endocrine system .

Pacreatic acinus : it is the main component of the exocrine secretory part of the pancreas. Pancreatic acini are cell groups forming the secretory units that release substances to be finally poured into the intestinal lumen. They are serous acini with cells showing purple cytoplasm, when staining with general dyes, such as haemotoxylin-eosin. The nucleus is rounded. The secretory products of serous cells are mainly digestive enzymes.

♣ Animal tissues: Pancreas .

Papilla: (in the tongue) it is bud on the surface of the tongue made up of epithelium and connective tissue. Regarding the morphology, tongue papillae may be filiform, fungiform, circumvallate and foliate. Fungiform and circumvallate papillae may content taste buds. Filiform papillae lack taste buds and function as a rough surface that help moving the food inside the mouth.

♣ Animal organs: Senses, Tongue .

Papillary dermis: it is the upper sublayer of the dermis, located just below the epidermis. Papillary dermis is loose connective tissue with many blood and lymphatic vessels that nurse the epidermis, as well as buffering the body temperature by vasodilation and vasoconstriction. In the papillary dermis, there are many nerve terminals from sensory neurons, some of them reaching the epidermis.

Parafollicular cell: (in the thyroid) (or C cell) it is a cell type found in the thyroid gland. Parafollicular cells release the calcitonin hormone when calcium concentration in blood is high. Calcitonin inhibits the calcium reabsorption in the bone tissue by influencing the osteoclasts activity. The bone extracellular matrix is the main calcium storage in the body.

♣ Animal tissues: Thyroid .

Parasympathetic ganglion: it is a type of peripheral ganglion (autonomic nervous system) generally found near the organs innervated by these ganglia, excepting the head and neck parasympathetic ganglia.

♣ Animal organs: Nervous system, Peripheral nervous system .

Parenchyma: (in plants) it is a type of tissue that performs many functions depending on on the location in the plant body. For instance, photosynthesis in the leaves, storing in stem and roots, gas conducting in the stem and root of some species, etcetera. So, parenchyma is named as photosynthetic parenchyma, aerenchyma, storing parenchyma, aquiferous parenchyma depending on the function it is performing. The typical cell of parenchyma tissue is the parenchymatic cell, that shows a thin primary cell wall. Parenchyma cells may undifferentiate, proliferate and differentiate again in other cell types of the plant. That is, they can regain meristematic activity.

♣ Plant tissues: Parenchyma, Photosynthetic parenchyma , Aeriferous parenchyma , Storing parenchyma , Aquiferous parenchyma .

Parenchyma cell : (in plants) (or parenchymatic cell) it is a type of cell that forms the parenchyma tissue, but is also present in other tissues. Together with meristem cells, parenchyma cells show a low level of differentiation. The main functions of parenchyma cells is to carry out photosynthesis, and also work as storage cells. They have the ability to undifferentiate and give rise to all cell types of a plant body. Most parenchyma cells show a primary cell wall. Parenchyma cells are distributed all over the plant organs, such as leave mesophyll, cortex and medulla of stems and roots, fruits and seeds. There are photosynthetic, storage, aquiferous and aeriferous parenchyma tissues.

♣ Plant tissues: Parenchyma, Vacular tissues, Protection, Support, Photosynthetic parenchyma , Aquiferous parenchyma , Aeriferous parenchyma , Storage parenchyma .

♣ Plant organs: Primary root , Secondary root , Primary stem , Secondary stem .

Pars plana: it is one component of the ciliary body of the eye. es una de las partes del cuerpo ciliar del ojo.

Pars plicata: it is one component of the ciliary body of the eye. es una de las partes del cuerpo ciliar del ojo.

Pectin: (in plants) it is a group of polysaccharides that are abundant in the primary cell wall and middle lamela of the cell wall. Pectins form a gel like substance that helps molecules to diffuse through tissues. Secondary wall lacks pectins.

♣ The cell: Extracellular matrix types, Cell wall .

Pedicel: (in plants) it is the short stalk connecting with the peduncle in compound flowers.

Peduncle: (in plants) it is the stald that connect the flower with the stem.

Perianth: (in plants) it is the part of the flower not in charge of producing gametes. Perianth is made up of sepals (calyx) and petals (corolla). It encloses the pistil and stamens.

Pericarp: (in plants) it is the part of the fruit that covers the seed. Pericarp consists in three layers: endocarp, mesocarp and exocarp. In dry fruits, it is difficult to distinguish these three layers. Pericarp is formed from the ovule walls of the flower.

Perichondrium: it is a layer of dense connective tissue covering some hyaline cartilages and the elastic cartilage. Perichondrium is divided into an external layer or fibrous perichondrium, and an internal layer or chondrogenic perichondrium. The chondrogenic perichondrium is the source of new chondrocytes, process that makes possible cartilage growing by apposition.

Pericicle: (in plants) it is a layer of cells immediately below the endodermis found in primary roots. Pericle cells are parenchymatic cells that are able to proliferate and differentiate in other cell types. In fact, lateral roots begin to differentiate from pericicle cells.

Periclinal: (in plants) it is a type of cell division where the new cell wall that separates the two daughter cells is parallel to the surface of the organ, usually a stem or a root.

♣ Plant tissues: Meristems , Protection.

Periclinal division: (in plants) it is a type of cell division where the new cell wall that separates the two new cells is parallel to the external surface of the organ, usually a stem or a root. Periclinal divisions allow the growth in thickness of the organ, therefore its secondary growth.

♣ Plant tissues: Meristems, Protection, Cork cambium, Vascular cambium .

Periderm: (in plants) it is a set of tissues that proctects the surface of stems and roots showing secondary growth. Peridermis replace epidermis and is formed after the differentiation of the secondary xylem and phloem. Peridermis consists in phelloderm, cork cambium (phellogen) and phellem (cork). The corck cambium gives cork autward and phelloderm inward.

Perilymph: it is the liquid that fills the scala timpani and scala vestibuli of the cochlea in the inner ear.

Perimisium: it is a layer of connective tissue that wraps muscle fascicles. A muscle fascicle is made up of many muscle cells, and several fascicles made a muscle.

Perineurium: it is the connective tissue that wraps small axon bundles that form the peripheral nerves.

Periostium: it is a layer of very fibrous connective tissue that wraps the outer surface of bones, excepting the joints, where it is the articular cartilage who covers the bone, and the tendon insertion points. The periosteum consists in an outer part conatining fibroblasts and an inner region with osteoprogenitor mesenchymal cells. The periosteum is highly irrigated by blood vessels and innervated by many nerve terminals.

Peripheral nervous system: it is the set of axons and neurons (usually forming ganglia) scattered through the animal body and outside the central nervous system. The peripheral nervous system can be divided into somatic and vegetative.

Perisperm: (in plants) it is a component of the nutritional storage tissues found in the seeds of some angiosperms. Perisperm is differentiated from the nucellus. In these seeds, both perisperm and endosperm are present (the endosperm is also storage tissue).

Peritrichial nerve ending: it is a type of free nerve ending associated with the hair follicles and involved in mechanoreception. Peritrichial nerve endins show fast adaptation, that is, they detect changes of mechanical stimuli, even the weakest, but not those that last in time.

Petals: (in plants) it is a part of the flower, more or less flattened, and showing vivid colors. Petals are highly modified leaves. Petals, together with sepals, form the sterile part of flowers. Petals and sepals form the so-called perianth. The corolla is the set of petals of a flower.

Petiole: (in plants) it is the peduncle that connect the stem with blade of the leaf. Petioles arise form the stem nodes. The axillary buds, that give rise to new branches, are found in the sharp angle formed between the petiole and the stem. Leaves not having petiole are known as sessile leaves.

♣ Plant organs: Leaf .

Phelloderm: it is a layer of cells that can be found in stem with secondary grow. Phelloderm is derived from the cork cambium and located just below this meristem. Phelloderm is made up of parenchyma cells with non-suberized cell walls. They can be distinguished from cortical parenchyma cells because they are piled in rows perpendicularly to the stem surface.

♣ Plant tissues: Meristems , Protection, Cork cambium , Periderm .

Phellogen: see cork cambium.

Phloem: (in plants) it is a type of plant vascular tissue specialized in conducting soluble organic molecules mainly produced by photosynthetic cells. Phloem is also known as sieve tissue. There are two types of phloem: primary and secondary. Primary phloem is derived from the procambium meristem. Protophloem is the initial stage and metaphloem is the final stage of the primary phloem differentiation. Those plants with secondary grow show secondary phloem, that differentiates from the vascular cambium. There are several types of cells in the phloem: sieve cells and sieve tubes are the conducting cells, and companion cells and sclerenchyma fibers are supporting non-conducting cells.

♣ Plant tissues: Vascular , Metaphloem , Secondary phloem , vascular cambium , Protection, Cork cambium.

♣ Plant organs: Introduction , Leaf, Pine leaf , Primary root, Monocot primary root , Dicot primary root , Tallo, Primary stem , Dicot primary stem .

Photosynthesis: it is a biochemical process where the carbon of the CO₂ is added to an organic molecule by using the energy of light. In plants, chloroplasts are responsible for getting energy from light thanks to chloropyll and a complex chain of protein located at the thylakoid membrane.

♣ The cell: Chloroplasts .

Photosynthetic parenchyma: (in plants) it is a type of parenchymatic tissue specialized in performing photosynthesis. Photosynthetic parenchyma cells contain a large amount of chloroplasts. This tissue is found in under the leaf epidermis, where the sunlight is more intense. It is also found in the green stem and branches, as well as in the aerial roots of some plant species.

♣ Plant tissues: Parenchyma, Photosynthetic parenchyma .

♣ Plant organs: Leaf, Pine leaf , Dicot leaf .

Phytohormone: (in plants) it is a type molecule released by plant cells that can affect distant cells in the body plant. Phytohormones influence the maturation, grow and metabolism of cells. Examples of phytohormones are auxin, gibberellins, ethylene, cytokinins and abscisic acid.

Pigmented layer: (in the eye) it is the outer layer of the internal wall of the eye. Pigmented layer differentiates from the encephalic evagination that produces the neuronal part of the eye. The retina is the inner layer of this evagination. However, pigmented layer is not composed of neurons but pigmented cells. Pigments prevent light dispersion that results in a more sharp vision. Pigmented layer is also involved in the homeostasis of the photoreceptors layer of the retina, with which it is in contact.

Pinnate leaf: (in plants) it is a type of compound leaf consisting in many leaflets arising from a common central axis.

♣ Plant organs: Leaf .

Pistil: (in plants) it is the part of flower where marcrospores are formed. The pistil is one or several modified leaves known as carpels. Each pistil consists in three parts: stygma, style, and ovary (where marcrospore is generated). The set of pistils of a flower is referred as gineceium.

Pith: (in plants) it is the region of the stem and roots found deeper than the vascular tissues. The pith is the center region of the organ and it is usually made up of parenchyma. Sometimes, the pith tissue degenerates and leaves an empty space.

♣ Plant organs: Introduction, Primary root, Secondary root, Primary stem, Secondary stem .

Platelet: it is a small portion of cytoplasm released by megacaryocytes in the bone marrow. Platelets are found in the blood and are involved in blood clotting when blood vessels are damaged. Platelets are present in mammals. In other vertebrates, there is a cell type called thromobocyte that performs a similar function than platelets.

Pleura: it is a sheet of tissue that form the thorax cavity of the body. Pleura consists of two layers of mesothelium (simple squamous epithelium): a visceral mesothelium closer to the lungs, and a parietal mesothelium closer to the thorax wall, ribs and intercostal muscles. Between both layers, there are serous liquid for lubrication.

Plumule: (in plants) is a structure of the plant embryo that will differentiate into the stem during seed germination. In the seed, plumule is found in the opposite site to the radicle, that give rises to the root.

Polar nucleus: (in plants) it is the nucleus found in the central region of the female gametophyte. There are two polar nuclei, which fuse between each other and with the generative nucleus of the pollen grain, and it develops into the triploid endosperm. Polar nuclei are also known as central nuclei.

♣ Plant organs: Seed, Flower .

Pollen: (in plants) it is the male gametophyte of plant flowers. Pollen grains are found in the anther of stamens, and consist in a coat (exin + intin) enclosing two haploid nuclei, which are the male gametes or microspores. One the nuclei, the vegetative nucleus, forms the pollen tube, and the other nucleus, the generative nucleus, divides in two. One of new generative nucleus fuses with the female haploid nucleus for fertilization, whereas the other forms the endosperm of the seed that nurtures the embryo. Pollen grains are extremely diverse in morphology.

Pollen sac: (in plants) it is the compartment that encloses the pollen in the stamens of flowers.

Pollinic tube: (in plants) it is the structure generated by the diploid cell of the pollen grain. The pollinic tube is initiated during pollination, when the pollen grain contacts with the stigma of the flower pistil. The pollinic tube grows from the stigma to the ovule of the ovary and transport the haploid nuclei to the micropyle of the ovule for fertilization.

Polyarch root: (in plants) it is a type of root with many rows (or poles) of protoxylem.

Pom: (in plants) it is a type of fruit where the endocarp shows cartilaginous consistence. Surrounding the endocarp, there is fleshy tissue from the receptacle of the flower (accessory fruits). An example is the apple.

Pore field: (in plants) it is a region of the cell wall of plant cells that shows groups of plasmodesmata. Pore fields are usually found in the thinner cell walls.

Porogamic fertilization: (in plants) it is a type of fertilization where the pollen tube enters through the micropyle of the ovary. It is the most frequent type of fertilization.

Portal system: it is a set of blood vessels that connect two capillary nets. Portal systems connect two organs. For instance, the intestine and the liver.

Postcapillary vein: it is a type of vein that gathers the blood from capillary networks. Postcapillary veins are narrows, up to 0.1 mm in diameter. The endothelium is very responsive to external stimuli and changes the adhesion strength between endothelial cells to let the lymphocytes to cross through and for the extravasation of blood serum. Postcapillary veins do not show a true tunica media.

Primary cell wall: (in plants) it is the layer of the cell wall found between the middle lamela and the plasma membrane, or between the middle lamella and the secondary cell wall. Primary cell wall is mainly composed of cellulose, hemicellulose and pectins. It is present in all plant cells and provides mechanical support, stiffness, cell shape, counteracts osmotic pressures, influences the growth of the cell, allows for molecule diffusion, and performs many other functions in the plant tissues.

♣ The cell: Extracellular matrix. , Carbohydrates, Cell wall .

Primary endosperm: (in plants) it is the endosperm in gymnosperm seeds. The primary endosperm is derived from a haploid nucleus of the female gametophyte. Therefore, it is a haploid tissue. The endosperm is a storage tissue found in seeds.

Primary growth: (in plants) it is the first type of growth in plants during shoots and roots development. Primary growth is produced by the activity of primary meristems. During primary growth, plants mostly grow in length. Some plants only show this type of growth, whereas others undergo a subsequent secondary growth.

♣ Plant tissues: Introduction, Vascular tissues, Protection, Protoxylem .

♣ Plant organs: Primary root , Primary shoot .

Primary meristem: (in plants) it is a type of meristem responsible for the growing in length of the plant body. Primary meristems may be named as apical caulinar (or apical shoot) meristem if it is located at the tips of stems and branches, or apical radical (or root apical) meristem if located at the tips of roots. Intercalary meristem are primary meristems located in the internodes of the stems and branches.

♣ Plant tissues: Meristems , Root primary meristem , Shoot primary meristm .

Primary phloem: (in plants) it is the first type of phloem during the plant grow. The first primary phloem is called protophloem, which is later replaced by metaphloem. Metaphloem is the permanent primary phloem type in those organs that do not show secondary grow. Metaphloem is replaced by secondary phloem during secondary grow.

♣ Plant tissues: Vascular tissues, Meristems , Metaphloem .

♣ Plant organs: Primary root , Secondary root , Primary stem , Pine leaf , Dicot primary root , Monocot primary root , Monoct secondary root , Dicot primary stem , Monocot primary stem , Dicot secondary stem , Gymnosperm scondary stem .

Primary root: (in plants) it is the root during the initial growing, primary grow, which is mostly an increase in length. In some plants, primary grow is the only type of growing. However, many plant species develop secondary roots with secondary grow, which is a growth to increase the root girth.

Primary xylem: (in plants) it is the first xylem to be differentiated in vascular plants. Initially, the protoxylem is formed, which is later substituted by the metaxylem. Metaxylem is permanent in those plants not showing secondary growth, but is replaced by secondary xylem in plants with secondary growth.

Procambium: (in plants) it is a primary meristem formed in embryos and adult plants. Procambium differentiate into primary xylem and phloem, as well as in the vascular cambium during secondary growth.

Progenitor cell: it is a type of undifferentiated cell that can divide and differentiate into functional cells. Progenitor cells have a limited proliferative capacity and all of them become differentiated. They are the intermediate step between stem cells and differentiated cells.

Proprioception: it is the process of sensing the inner parts of the animal body. Proprioception information is related to the position of the body and its components, balance and movement.

Proteoglycan: it is a type of complex molecule composed of a polypeptide and a variable number of glycosaminoglycans. Proteoglycans are found in the extracellular matrix of animal tissues. There are many types of proteoglycans regarding the amino acid chain, and the number and type of glycosominoglycans. Different types of extracellular matrices contain a different composition of proteoglycans.

Protodermis: (in plants) it is a layer of cells derived from the apical meristems of both shoots and roots that differentiates to become the epidermis.

Protophloem: (in plants) it is the first type of phloem to be formed and functional during the primary growth of stems and roots. Protophloem is replaced by metaphloem as the plant grows.

Protoxylem: (in plants) it is the first functional xylem formed in shoots and roots. Protoxylem is replaced by metaxylem as the plant grows..

Pseudostratified epithelium: it is a type of covering epithelium where all cells contact with the basal lamina, but not all of them are part of the free surface of the epithelium. Therefore, cells look like they are located at different height in the epithelium, so that the epithelium appears to be stratified, but it is not. For instance, tracheal epithelium is pseudostratified epithelium.

♣ Animal tissues: Epithelium , Pseudostratified epithelium .

Pupil sphincter: it is the muscle that closes the pupil in the eye.

Pteridophyte: it is a group of vascular plants that does not produce seeds. Pteridophytes are ferns and alike. They are also known as crytogams.

Pit: (in plants) it is an interruption of the secondary cell wall for communicating adjoining cells. Pits are abundant in tracheids and allow the flux of substances. Primary pits are those with the interruption is also extended to the primary cell wall and areolated pits when there is a pit ledge on the secondary cell wall.

Pulmonary artery: it is one of the arteries emerging from the heart and carries the blood toward the lungs.

Pupil: it is the opening in the iris of the eye.